fix PR8753, eliminating a case where we'd infinitely make a
substitution because it doesn't actually change the IR. Patch by
Jakub Staszak!
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@121361 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/Scalar/MemCpyOptimizer.cpp b/lib/Transforms/Scalar/MemCpyOptimizer.cpp
index 318e212..6f93e32 100644
--- a/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ b/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -675,6 +675,14 @@
if (M->getSource() != MDep->getDest() || MDep->isVolatile())
return false;
+ // If dep instruction is reading from our current input, then it is a noop
+ // transfer and substituting the input won't change this instruction. Just
+ // ignore the input and let someone else zap MDep. This handles cases like:
+ // memcpy(a <- a)
+ // memcpy(b <- a)
+ if (M->getSource() == MDep->getSource())
+ return false;
+
// Second, the length of the memcpy's must be the same, or the preceeding one
// must be larger than the following one.
ConstantInt *C1 = dyn_cast<ConstantInt>(MDep->getLength());